As The Mail reports,the attack came as Raheem Kassam, a former aide to Mr Farage, insisted the party was beset by back-stabbing and duplicity.

‘I, like ordinary Ukip members, am so tired of the games that are being played at the top of this party. There is so much corruption. There is so much duplicity. There are so many people shaking hands with one another and then knifing them in private. It has to stop,’ he said.

Mr Woolfe has admitted he had flirted with defecting to the Tories after being barred from the last leadership race by the party’s NEC.

A statement from UKIP interim leader Nigel Farage said:

I deeply regret that following an altercation that took place at a meeting of Ukip MEPs this morning that Steven Woolfe subsequently collapsed and was taken to hospital.

His condition is serious.

The Telegraph quotes sources as saying Woolfe was punched by a colleague during a meeting of UKIP members of the European Parliament in Strasbourg. Mr Woolfe is reportedly suffering from bleeding of the brain after he was punched. One witness said he fell into a window after being punched.

Suzanne Evans, a leading contender to be Ukip leader, said: ‘Shocked to hear Steven Woolfe has apparently collapsed in the European Parliament in Strasbourg. Wishing him well for a speedy recovery.’

Former Farage spin doctor and outsider for the top job Raheem Kassam said: ‘I have cancelled my appearance on BBC @daily_politics out of respect to @Steven_Woolfe who is currently being treated after collapsing.’